Fast-fill, or quick take-up, master cylinders have been used in braking systems of automotive vehicles for some time. Concerns about the performance of such master cylinders include brake pedal travel variations as a function of the application rate of braking force. Another concern involves the inconsistency of pedal feel during transitions from fast-fill pressurizing chamber to standard master cylinder pressurizing chamber fluid delivery.
In a braking system using a standard master cylinder, as force is applied to a brake pedal, a piston within the master cylinder begins to move, increasing the pressure of brake fluid within the braking system. The ratio of pedal effort, or braking force, with respect to pedal travel is relatively low and increases relatively slowly as brake pads or shoes are forced from retracted positions to positions where they make contact with respective brake disks or drums.
The pedal effort that must be exerted to obtain additional pedal travel increases more rapidly thereafter. The increases in brake fluid pressure and in pedal effort are both functions of increases in pedal travel, and both pedal effort and brake fluid pressure increase smoothly in a substantially coincidental and generally exponential manner. In a braking system using a standard master cylinder, the amount of pedal travel is not a function of the rate at which pedal effort is applied.
In a braking system using an existing fast-fill master cylinder, a flow restriction valve is used to provide an alternate path for brake fluid from a fast-fill pressurizing chamber to a brake fluid reservoir. The flow rate of brake fluid through such a flow restriction device is a function of pressure. If force is applied to a brake pedal so rapidly that brake fluid pressure increases before any appreciable amount of fluid can flow through the restriction, pedal travel is relatively minimal.
If, however, force is applied to the brake pedal so slowly that fluid is capable of flowing through the flow restriction device without any appreciable increase in brake fluid pressure, pedal travel can be considerable. Pedal travel in a braking system using an existing fast-fill master cylinder is therefore a function of the rate at which braking force is applied to the pedal.
In the braking system just described, a relief valve is disposed between the fast-fill pressurizing chamber and the brake fluid reservoir of the master cylinder so that, after the brake pads or shoes have been rapidly forced into contact with respective brake disks or drums and the increased brake fluid flow used to accomplish this is no longer needed, the pressure that is then rapidly increasing in the fast-fill pressurizing chamber, with its corresponding rapid increase in pedal effort, can be relieved.
When the relief valve opens, however, the brake fluid pressure and corresponding pedal effort are relatively high. The smooth increase in brake fluid pressure is interrupted, the rate of its increase being sharply reduced. This results in an inconsistent pedal feel during this transition from fast-fill pressurizing chamber to standard master cylinder pressurizing chamber fluid delivery. Following this period, the pedal feel would approximate that of a braking system using a standard master cylinder.
While braking systems using existing fast-fill master cylinders function with a certain degree of efficiency, none disclose the advantages gained by using the improved fast-fill master cylinder of the present invention as is hereinafter more fully described.